1. Field of the Invention
The invention relates generally to antifouling marine coatings for the submersible surfaces of boat hulls, pilings, buoys, floating or emplaced offshore platforms, submergence vehicles, navigational aids, and any marine structures where marine biofouling may be a problem. More particularly, the invention relates to antifouling marine coatings which include an antibiotic active agent.
2. Description of the Related Art
Antifouling marine coatings have been used for many years to paint the hulls of ships to prevent the attachment of fouling organisms such as barnacles and algae. Most antifouling coatings include a marine biocide which is at least slightly soluble in seawater and which is slowly and continuously introduced into the adjacent seawater over the useful life of the coating to kill or weaken fouling organisms which would otherwise attach themselves to the ship hull.
U.S. Pat. No. 4,678,512, issued Jul. 7, 1987 to Grams, describes a marine antifouling coating for surfaces contacted by seawater, having dispersed therein an effective amount of particles of a polycyclic naphthacenecarboxamide which serves as an active antifouling agent. Many of these particles are near the interface of the antifouling coating and the seawater and are continually leached by the seawater to provide an inhibitory action against marine life attaching to the coating. Also, as the coating is worn, eroded or abraded, new particles are exposed to and leached by the seawater.
U.S. Pat. No. 4,286,988, issued Sep. 1, 1981 to Castelli et al, describes a copper base antifouling paint in which a Cu.sub.2 O or Cu.sub.2 S biocide for marine fouling organisms is slowly leached from the paint into the adjacent seawater The paint also includes ammonium sulfate to reduce or prevent the precipitation of certain water insoluble copper salts, which, in turn, reduces the frequency at which ships must be scraped to renew the old antifouling coatings or repainted.
U.S. Pat. No. 4,675,051, issued Jun. 23, 1987 to Baxter, describes marine antifouling paints, each of which includes a film-forming binder which is gradually dissolved in seawater, a marine biocide, and a sparingly soluble pigment which may also be the marine biocide of the paint. These antifouling paints are smoothly dissolved from a ship's hull in service and allow a steady release of the biocide in the paint. Biocide pigments which may be used alone or in combination in these antifouling paints include sparingly soluble copper and zinc compounds, such as cuprous oxide, cuprous thiocyanate, zinc oxide, zinc chromate, zinc ethylene bis(dithiocarbamate), zinc dimethyl dithiocarbamate and zinc diethyl dithiocarbamate.
U.S. Pat. No. 4,865,909, issued Sep. 12, 1989 to Manniso, describes an antifouling marine coating which includes a hydrophobic microporous polymeric membrane adhered to a surface by a marine paint containing a biological toxicant known to be effective against fouling organisms, e. g., cuprous oxide. With no marine organism present on it, the microporous membrane is hydrophobic to seawater and the pores are small enough so that no water enters the membrane to contact the toxicant. When a fouling organism attaches, it penetrates the membrane pores, contacts the toxicant in the paint which has partially filled the pores, is weakened or dies, and is washed off the membrane, which again becomes hydrophobic to seawater.
The use of a tin compound as the biological toxicant in antifouling marine paints is mentioned in the above-cited Manniso U.S. Pat. No. 4,865,909 as well as in numerous other patents and publications, even though tin compounds have been abandoned as antifouling compounds by the U.S. Navy and others because of tin's extreme toxicity to fish and because of the health problems induced by tin in persons dealing with these tin compounds. However, copper has been used as a fouling deterrent since Roman times, and does not appear to be particularly hazardous to persons applying it to ship bottoms. For these reasons, most antifouling marine paints prior to the present invention have been copper-based paints, with cuprous oxide being the most widely used active antifouling agent.
In a review article entitled Structure and Function of the Cell Envelope of Gram-Neqative Bacteria by J. W. Costerton, J. M. Ingram, and K. J. Cheng, published in BACTERIOLOGICAL REVIEWS, Vol. 38, No. 1, p. 87-110, March, 1974, it is noted that gram negative microorganisms possess a complex, multilayered cell wall, including a lipopolysaccharide (LPS)-containing outer membrane, an inner, cytoplasmic membrane, and a periplasmic space between the outer and inner membranes, with each layer including different enzymes. The outer membrane and its associated LPS function as a "molecular sieve" to prevent the passage therethrough of many antibiotics and other potentially harmful molecules. The ion exchange effect of the multiple layers of the cell wall also protects the cell by binding various ions and molecules. In addition to degradative enzymes for breaking down complex food molecules, gram-negative cell walls contain other enzymes which may be used to localize potentially toxic autotrophic reactions within the cell wall. It is suggested that inorganic substrates such as iron may be oxidized while in a complexed form in the cell wall. Because these enzymes are retained in layers of the cell wall outside the cytoplasmic membrane, their products may be readily transported into the cell or allowed to diffuse into the medium.
The period of time that oonventional oopper base antifouling paints are effective in preventing the attachment and growth of marine fouling organisms to ship hulls varies greatly, from about 12 months to 36 months, after which time the hull surface must be scraped and brushed to remove marine fouling organisms and materials deposited thereon. Old antifouling coatings can often be renewed by such hull cleaning operations, and only touchup painting is necessary. However, even with such periodic cleaning and touchup, the copper base antifouling paints used on ship hulls by the U.S. Navy must be completely replaced at least every five years.